The system includes an approach sensor. Such a sensor measures the position of an object relative to three reference axes and in a given volume, said object having one or more passive references fixed thereto. Approach sensors are used, in particular, in space, i.e. under conditions of weightlessness, when performing docking or stowage maneuvers with two space vehicles. The measurements performed generally include:
an angle measurement; PA1 a distance measurement; and PA1 a speed measurement.
These measurements are generally performed with an accuracy of about:
.+-.1.degree. for the angle measurement; PA0 .+-.5 mm for the distance measurement (at a range of 1 m); and PA0 &lt;10.sup.-2 m/s for the speed measurement.
At the beginning of the approach stage, the object to be located must already be in a certain volume which is defined relative to the reference axes. Several types of sensor can be used to achieve the desired results:
Optical sensors:
Optical sensors may be of the imaging type or of the telemeter type.
Imaging type sensors measure the angle at which the passive reference mark on the object is seen, and also measure variation in said angle. Such sensors are thus very complex to build because of the complexity of the target patterns and also because of the difficulty associated with processing the information digitally (image analysis). Such imaging sensors appear to be more suitable for performing a portion only of the required functions.
Telemeters are based on the principle of measuring a characteristic of a light beam or pencil, with the desired measurement being deduced from the measured characteristic. Four such techniques are used:
(1) The light flux is measured as reflected from the target pattern on the object. The drawbacks of this method are its sensitivity to interfering illumination and the inability of the system to perform absolute measurements;
(2) The frequency of the light wave reflected from the pattern is measured, and deductions are made based on the Doppler effect. The drawbacks of this method include the high degree of frequency stability which is required and the fact that it can only be used to measure radial speeds;
(3) The transit time of a pulse reflected from a target on the object is measured. The major drawback of this technique is the highly complex electronics which need to be associated therewith in order to obtain a high degree of accuracy; and
(4) The phase shift on an intensity modulated light beam is measured. These techniques do not give the necessary accuracy at short distances.
Microwave sensors:
Such systems are similar to radars, anti-collision systems, airport radars, etc. . . Various techniques can be used with microwaves. However, depending on which technique is used, one type of measurement will be facilitated relative to the others. The drawbacks of microwave systems are the bulk and the complexity of the installation and the possibility of HF interference with other equipment in the vicinity. There is also the problem of poor performance at short distances.
Preferred embodiments of the present invention provide a system for locating an object and including an approach sensor which belongs to the family of imaging sensors using special passive targets fixed on the object being approached.